Electro-optically active devices have conventionally been prepared using lithium niobate (LiNbO3). As is familiar to those of ordinary skill in the art, lithium niobate has a lattice structure in which the lithium ions have a non-centrosymmetric position. In the absence of an electric field, this non-centrosymmetric position imparts the material with a net polarization. Applying an electric field to the lithium niobate shifts the position of the lithium ions, changing the net polarization, and refractive index, of the material. Thus, the phase of light propagating through the waveguide may be altered by applying an electric field to the waveguide.
FIGS. 1A and 1B schematically illustrate an exemplary electro-optically active device 100 that includes lithium niobate substrate 101 in which waveguide 102 is formed, input optical fiber 111, output optical fibers 112, and voltage generator 121. Waveguide 102 may be formed by exchanging protons for some of the lithium ions in the substrate 101 within defined areas, e.g., by immersing the substrate into a solution containing a proton exchange acid, such as benzoic acid. The proton-exchanged areas have a higher extraordinary refractive index than the remainder of the substrate, and so act as a waveguide 102 that transports light through substrate 101 with relatively low loss. In the illustrated device, light is introduced to waveguide 102 through input optical fiber 111. Junction 105 of waveguide 102 divides the light into two portions and respectively guides the light portions into sections 106 and 107 of waveguide 102. Electrodes 122 are positioned on either side of the waveguide sections 106, 107, and separated from the waveguide sections by buffer regions 103. In one example, the inner edges of the electrodes are spaced approximately 10 microns from each other.
Voltage generator 121 is programmed to independently apply voltages to different pairs of the electrodes 122, so as to change the phase of the light traveling through the waveguide section adjacent that pair. For example, as illustrated in FIG. 1B, waveguide section 106 has a net polarization 104 in the absence of an electric field. Voltage generator 121 (not shown in FIG. 1B) may apply a voltage across electrodes 122, which generates an electric field along the crystallographic z-axis to change the net electrical polarization 104 of waveguide section 106, which induces a phase change of light traveling through that section. The magnitude of the change in the material's net polarization, and the magnitude of the phase change, may be proportional to the applied electric field. The light in waveguide sections 106, 107, may be coupled out of waveguide 102 and into separate output optical fibers 112.
Alternatively, in the electro-optically active device 100′ illustrated in FIG. 1C, the light in sections 106′, 107′ of waveguide 102′ may instead be recombined at junction 108′, where they interfere with one another. Because the relative phase of the light portions traveling through waveguide sections 106′, 107′ may be controlled via voltage generator 121′, the intensity of the light at junction 108′ may be modulated as desired. For example, if the portion of light in section 106′ is phase delayed by an even multiple of π relative to that in section 107′, then the two portions of light will constructively interfere with each other, yielding maximum brightness. Or, for example, if the portion of light in section 106′ is phase delayed by an odd multiple of π relative to that in section 107′, then the two portions will completely interfere with each other, yielding minimal brightness. Any intensity in between may be selected by adjusting the relative phase delays via voltage generator 121′. The output of waveguide 102′ is coupled into a single output optical fiber 112′. Configurations such as that illustrated in FIG. 1C may be referred to as a Mach-Zehnder modulator. Waveguide configurations other than those illustrated are also common.
Lithium niobate waveguides prepared using conventional proton exchange techniques are vulnerable to performance degradation, limiting their application. For example, as illustrated in FIG. 2, a waveguide 202 may be prepared by contacting a substrate 201 with a solution 211 that contains protons but does not dissolve lithium niobate. Near the upper surface of the substrate, protons diffuse into the lithium niobate lattice, displacing lithium ions which then become dissolved in the solution 211. The proton exchange is allowed to proceed until the protons have sufficiently penetrated substrate 201 to form waveguide 202, which has a refractive index suitable for containing light with relatively low loss, e.g., as described above with respect to FIGS. 1A-1C.
However, the resulting waveguide may be unstable due to stresses caused by the ion exchange process. These stresses may damage the waveguide by causing crystal dislocations and/or cracks. For example, FIG. 3A is a transmission electron microscope image of a lithium niobate waveguide prepared by immersing substrate 301 into undiluted benzoic acid at 230° C. for one hour. Proton exchanged layer 302, which has a thickness of about 1.3 microns and forms the top portion of substrate 301, contains numerous crystalline defects 303, visible as parallel lines. Such defects may arise, for example, because protons are smaller than the lithium ions they replace, causing the crystal lattice to shrink. The sharp interface between the proton exchanged layer 302 and remainder of the substrate 301 corresponds to a change in lithium niobate phase. Because the transition between waveguide 302 and substrate 301 is abrupt, the mismatch of the crystalline lattices may induce stress and cause the defects 303 apparent in FIG. 3A. FIG. 3B is an electron beam diffraction pattern from a sample similar to that shown in FIG. 3A. As can be seen, the diffraction pattern includes several bright peaks 311 that are spaced far apart from one another, corresponding to the regular crystalline lattice. There are also numerous dimmer peaks 312 in the pattern that are spaced closer to each other, corresponding to large-scale “superlattice” defects in the crystalline lattice, with a lattice constant that is about 5 times larger than the basic lithium niobate lattice, demonstrating that a different phase has been formed.
Any defects in the waveguide may serve as electron donors, proton diffusion pathways, and/or proton accumulation sites, which may detrimentally affect both the electrical insulation and the stability of the waveguide material. Due to this instability, the performance of the crystal may degrade over time, including gradual changes in refractive index and/or electrical conductivity. As illustrated in FIG. 2, such degradation may be reduced by annealing the waveguide before use, which may heal coarse lattice defects and convert the relatively thin waveguide 202 into a thicker waveguide 204 having a lower proton concentration and a more gradual transition between the crystal lattices of the waveguide 204 and the underlying substrate 201. Such annealing may also cause lateral diffusion of the waveguide, reducing the width of the buffer between the waveguide and the electrodes discussed above with respect to FIGS. 1A-1B. Such annealed waveguides are still susceptible to degradation under normal operating conditions, albeit much more slowly than non-annealed waveguides.
Drifts in the refractive index of the waveguide may be electronically compensated for by monitoring the phase and/or intensity of the light output from the waveguide, and adjusting the magnitude of the electric fields to achieve similar performance. For example, by increasing the voltage, a satisfactory response may be obtained; however, once the degradation exceeds the ability of the electronics to compensate for such drift, the waveguide may no longer be capable of performing to specification. Compensating for drifts in the electrical conductivity of the waveguide may also be difficult to achieve. Lithium niobate is a very good insulator, having a conductivity of less than 10−12 (Ωcm)−1. Disturbances of the crystalline lattice cause a substantial increase in conductivity, which allows space charges to accumulate near the electrodes, producing a drift condition. The feedback circuitry required to adjust the applied voltage to compensate for drift in the refractive index and electrical conductivity of the waveguide may add significant expense and complication to the modulator circuit, may limit the switching speed of the device, and may limit the environments in which the device may be used.
U.S. Pat. No. 7,170,671 to Wu et al. discloses a method of forming waveguides that includes exposing a lithium niobate crystal to a diluted proton exchange step, followed by a reverse proton exchange step. Specifically, Wu discloses diluting benzoic acid, a proton exchange medium, by adding lithium benzoate, and applying the diluted solution to the crystal surface at a temperature of 300-380° C. for several tens of hours. Wu discloses that such processing provides a crystal having a single phase. Wu discloses that a high temperature anneal of the crystal after the proton substitution could damage the crystal, and that it is advantageous that the method eliminates the need for such post exchange heat treatment. Wu discloses that the refractive index of the optical waveguide region may be further shaped and rendered symmetric by following the proton exchange using a reversed proton exchange (RPE) method. Although Wu alleges that a waveguide of very high quality may be obtained using these two steps, the high-temperature diluted proton exchange requires exposing the crystal to a temperature well in excess of the atmospheric boiling point of benzoic acid, which is about 250° C. Because benzoic acid would otherwise be in a gaseous phase at the reported temperatures of 300-380° C. at atmospheric pressure (1 atm), it can reasonably be inferred that Wu's method requires pressurizing the heated proton exchange solution well above atmospheric pressure to maintain it in a liquid phase during the reaction. Such pressurization may be both inconvenient and dangerous, particularly over extended periods of time.